1. Field of the Invention
The present invention relates to an ink jet recording apparatus, specifically to an ink supply recovery for supplying ink from a main-tank to a small sub-tank for directly supplying ink to a recording head at a predetermined timing.
2. Description of the Related Art
There is an ink jet recording apparatus of a serial scanning system for realizing a smaller sized apparatus. The ink jet recording apparatus of a serial scanning system is provided with a carriage movable in a main scanning direction. Further, an ink jet recording head as recording means and an ink tank as an ink container are mounted in a replaceable manner to the carriage. An image is recorded on a recording medium by the repetition of the main scanning of the carriage and the sub scanning of the recording medium.
Such a recording apparatus may be communicated or integral with PDA (personal digital assistants) or a camera to output the image. In this case, it is desirable to further minimize a size of the carriage or a capacity of the ink tank or ink. However, if the capacity of the ink tank is excessively small, the frequency of the replacement of the ink tank increases, and in an extreme case, it is necessary to replace the ink tank during the recording operation, which is not so practical.
To solve such a problem, the following content has been proposed, for example, in Japanese Patent Application Laid-Open No. 2000-334982. That is, a system is disclosed, including an ink storage member having a relatively small capacity provided in the carriage (hereinafter referred to as a sub-tank) and an ink storage member having a relatively large capacity provided at a fixed position of the apparatus (hereinafter referred to as a main-tank), in which ink is replenished from the main-tank to the sub-tank at a suitable timing (which system is referred to as “an on-demand supply system”. According to the on-demand supply system, it is possible to use the main-tank having a far larger ink capacity than the sub-tank on the carriage. Thereby, the replacement of the ink tank does not occur so frequently. Also, even if the ink capacity of the sub-tank is small, it is possible to replenish ink to the sub-tank at a suitable timing. Accordingly, the problem in that no ink exists in the sub-tank during the recording operation could be solved.
However, in the prior art on-demand supply system, there is a problem described below when the ink jet recording apparatus has been left unused for a long period. That is, a volatile component in the ink stored in the sub-tank is vaporized to increase the concentration of ink colorant. If the concentration of the ink colorant increases, a color tone of an image to be initially recorded next time becomes unnatural, which differentiates the color tone between a plurality of sequentially recorded images. Such a phenomenon is more significant as the environment in which the recording apparatus is left is lower in humidity. This problem is particularly unfavorable in a recording apparatus for a camera for recording a photograph.
Several countermeasures have already been proposed to solve the above-mentioned problem. For example, it is considered as one of countermeasures to provide a mechanism for closing an opening of the sub-tank if necessary. Another is considered, which is to use a material having a low gas-permeability for the sub-tank. Further, another method is considered in which a thickness of the tank may be increased. In either of the countermeasures, it is possible to reduce the evaporation to a certain extent. However, these countermeasures are not essential but may be a so-called symptomatic treatment expecting the prolongation of life unless the evaporation becomes zero. Also, these countermeasures result in the cost-up and the enlargement of the size of the sub-tank as well as disturb the miniaturization of the recording apparatus. Thus, these countermeasures have not completely been effective.
Also as the other countermeasure, ink may be once withdrawn from the sub-tank after the recording operation has finished. According to this method, the next recording operation can be carried out at a stable concentration of colorant of the ink stored in the main-tank, irrespective of the colorant concentration of the ink left in the sub-tank. However, the present inventors have confirmed that a desired effect may not obtained in some cases when the relationship is uncertain among a size of the sub-tank, an amount of residual ink and the composition of ink used. The method of discharging ink from the sub-tank after recording operation is not known to the public. Also, the following problems which occur when this method is employed, of course, are not known to the public too.
Problems inherent to the above method will be described below with reference to the attached drawings.
FIGS. 1A to 1D are an illustration for explaining the relationship between the sub-tank and ink in the sub-tank in the on-demand supply system.
FIG. 1A represents the sub-tank when the recording operation has finished. An ink absorber B401 is filled in the sub-tank B400 for generating a negative pressure for sucking ink therein. In this example, a sponge is used for the ink absorber B401.
In FIG. 1A, a medium amount of ink is left in the ink absorber B401.
FIG. 2B represents the sub-tank B400 immediately after the ink has been withdrawn there from a state shown in FIG. 1A. While the ink is withdrawn, the ink once absorbed in the ink absorber B401 is not completely removable. Ink such as one adhered to sponge fibers is left as it is.
FIG. 1C represents a state of the sub-tank left for a little while in the state shown in FIG. 1B. In this state, a volatile component of the ink left in the sub-tank B400 is vaporized to some extent. Therefore, an amount of ink is less than that in the state shown in FIG. 1B. However, only the volatile component is vaporized, the concentration of the colorant in the ink becomes rather higher.
FIG. 1D represents a state of the sub-tank again filled with ink for the next recording operation. In this state, ink of normal concentration is filled in the sub-tank shown in FIG. 1C in which the ink with high colorant concentration is left. Accordingly, the colorant concentration of ink becomes somewhat higher than in the initial state.
To understand such a state in more detail, the present inventors have made a sub-tank of a conventional on-demand supply system and studied the relationship the colorant concentration and the image by using this sub-tank. The content and result of this study performed by the present inventors will be described below.
First, polypropylene fibers having a bulk volume of 0.01 cc was filled as a sponge in the sub-tank having the capacity of 0.1 cc. As described with reference to FIGS. 1A to 1D, the operation was repeated for “removing the residual ink from the sub-tank after the recording operation, and after the sub-tank is dried, filling fresh ink again therein and a recording an image”. As a result, after such an operation was repeated a plurality of times, an image of an unnatural color tone was obtained because the colorant concentration of ink is too high. Further, when the same images were sequentially recorded, it has been confirmed that the difference in color tone becomes significant between the plurality of images.
The present inventors have strictly measured an amount of ink in the sub-tank in each of processes in the repetition of the operation. It is ideal that an amount of ink obtained by subtracting a volume of the sponge from a capacity of the sub-tank; i.e., 0.1−0.01=0.09 (cc) is filled in the sub-tank when the ink is initially filled in the ink. In practice, however, it has been confirmed that the amount of ink is of the order of 0.08 cc. This is because air initially existing in the sponge is left in the ink as micro-bubbles (dead air). It has been confirmed that a volume of the dead air is approximately 0.01 cc. Accordingly, the amount of ink which could be filled in the sub-tank has been 0.1 (the capacity of the sub-tank)−0.01 (the bulk volume of sponge)−0.01 (the amount of dead air)=0.08 (cc).
In the state shown in FIG. 1B in which the ink has been sucked out from the sub-tank, it has been confirmed that an amount of residual ink which was not discharged or adhered to the sponge is approximately 0.02 cc. This amount of ink does not disturb the formation of image in the usual ink jet recording apparatus having the ink tank of several cc to several tens cc. However, when the apparatus is of the on-demand supply system using the sub-tank of 0.1 cc, this value cannot be neglected.
In this regard, when the composition of ink used in the above-mentioned study is represented by a weight ratio, a volatile component (which can be vaporized) such as water or isopropyl alcohol is 0.7, a component difficult to be volatile such as glycerin is 0.25, and the colorant concentration is 0.05. That is, this ratio is the initial condition of the ink in the sub-tank. As a result of repeating the “recording, withdrawal of ink and filling of ink” thereafter, it was confirmed that the colorant concentration reaches a saturated value of approximately 6.1%. This value is 1.2 times or higher than the initial value of 5%. The “saturated value” referred to in this text represents a value of the colorant concentration in the sub-tank which is never exceeded even if the above operation is repeated. Therefore, this is different from the chemical “saturated value” of the original meaning. In this text, for representing such a state, a term “saturated value” or “saturated state” is conveniently used.
Even if the increase in colorant concentration of ink occurs, the above-mentioned countermeasures are effective provided this value is suppressed beneath a level at which no problem arises in the recorded image. However, when the control of the colorant concentration during the repeated operation is uncertain and the colorant concentration is not within a predetermined range, the rise of the colorant concentration of ink causes the trouble on the recorded image.